A distribution power combiner of such a type as explained in the Microwaves and RF vol. 8, pp107-112 1998 has been known as a power combiner applied to a power amplifier of a cellular base station. FIG. 18 shows a circuit configuration of the conventional distribution power combiner. A combining number will be defined as n. A RF signal applied from an input terminal 7 is distributed to respective amplifiers 101-1 through 101-n by branch circuits 103-1 through 103-(nxe2x88x921) in sequence. The RF signals amplified by the respective amplifiers 101 are sequentially combined into one by combiners 102-1 through 102-(nxe2x88x921), after which the combined one is outputted to an output terminal 8.
In order to bring out the full linearity of a semiconductor device used in each amplifier 101, it is desirable to reduce the maximum power inputted to each amplifier 101, i.e., equalize respective power inputted to the respective amplifiers 101. At this time, the power to be amplified by the respective amplifiers 101 become equal to each other.
Power distribution ratios for equalizing the power inputted to the respective amplifiers 101 are represented every branch circuits. If power inputted to an input transmission line 106 from the input terminal 7 is defined as n, it is then necessary to distribute power to be inputted to the first amplifier 101-1 and power to be inputted to the second branch circuit 103-2 in a ratio of 1:(nxe2x88x921). Distribution ratios to other branch circuits are determined in a similar relationship.
On the other hand, if power outputted from the output terminal 8 through an output transmission line 107 is defined as n, it is then necessary to combine first power outputted from the nth amplifier 101-n and (nxe2x88x921)th power outputted from the nxe2x88x921th combiner 102-(nxe2x88x921) into one. Distribution ratios to other combiners are determined in a similar relationship.
Such a prior art is accompanied by a problem that compensation for the linearity of each amplifier 101 is not taken into consideration and a power component of harmonic components caused by distortion developed in each amplifier is outputted from the output terminal 8 of the power combiner. Thus, in the conventional power amplifier, a filter for cutting off these harmonic components must be inevitably connected to a stage subsequent to the power combiner. The efficiency of power has been impaired as the entire power combiner including the filter due to an insertion loss produced by the insertion of the filter.
In the aforementioned prior art, the branching and combining of power have been performed by directional couplers. As the prior art that performs the branching and combining of the power, there is known one utilizing impedance ratios between lines in addition to the directional couplers. The branching and combining of the power are performed according to the impedance ratios between the lines. If the first branch circuit 103-1 is explained by way of example, then the characteristic impedance of a line extending from the input terminal 7 to a first connecting point, the characteristic impedance of a line extending from the first connecting point to the first power amplifier, and the characteristic impedance of a line extending from the first connecting point to the second branch circuit (second connecting point) may be defined as Z0/n, Z0 and Z0/(nxe2x88x921) respectively.
However, when the branching and combining of the power are performed according to the impedance ratios between the lines, isolation is insufficient between the plurality of amplifiers 101 while the circuit can be constructed at low cost as compared with the use of the directional couplers. Therefore, there is the potential that since the relationship in impedance between the distribution power amplifiers are disturbed when characteristic changes occur in any or more of the amplifiers 101, the efficiency of power combining will be greatly impaired. There is also a possibility that if one amplifier develops trouble and the input/output impedance thereof becomes infinite, then the output impedances of other amplifiers rise and the power reflected onto each amplifier increases, thus causing trouble that the amplifiers will be destroyed due to the reflected power on a chain reaction basis.
In a travelling wave power combiner, power are not combined into one if power outputted from respective amplifiers are not kept in balance. In the travelling wave power combiner according to the present invention, harmonics outputted from amplifiers are canceled out while the power outputted from the respective amplifiers are being kept in balance.
The principle of the present invention is shown in FIGS. 19A and 19B. The lengths (electric lengths) of transmission lines for connecting between branch circuits or between combiners will be defined as L (for simplification, the branch circuits and combiners are omitted in the drawing). FIG. 19A shows the relationship between the differences in phase between kth harmonics developed in an output point 1901 and line lengths L for canceling out the phase differences. The wavelength of a fundamental wave is defined as xcex and a phase shift of xcfx86 will be developed due to each line length L. If the phase of an input point 1900 is defined as the reference, a kth harmonic developed from a first amplifier 1902 produces a phase shift of kxcfx86 at the output point 1901 through a transmission line 1905. On the other hand, a kth harmonic produced from a second amplifier 1903 produces a phase shift of xcfx86 at the output point 1901 through a transmission line 1904. Thus, the phase difference xcex94xcfx86 at the output point 1901 between the kth harmonics developed in the two amplifiers results in (kxe2x88x921)xcfx86. When the phase of the kth harmonic from the first amplifier 1902 and the phase of the kth harmonic from the second amplifier 1903 are reversed, their kth harmonics are canceled out. In the case of xcex94xcfx86=xcex/2, the phases of the kth harmonics from the two amplifiers are inverted. Line lengths in this case are shown so as to correspond to second to 5th harmonics.
On the other hand, it is necessary to keep the power outputted from the respective amplifiers in balance with a view toward functioning as the travelling wave power combiner. This means that the combining or cancellation of the power (fundamental waves or harmonics) outputted from the respective amplifiers need to be equally executed for all the amplifiers. If, for example, harmonics from only some of the amplifiers in the travelling wave power combiner are canceled out, there is then a difference in magnitude between fundamental-wave outputs of the harmonics-canceled amplifiers and the amplifiers free of the cancellation of the harmonics, whereby the combiner is kept off-balance.
In order to make combinations of the amplifiers for canceling the harmonics, the number of the amplifiers contained in the travelling wave power combiner is set to a even number. Examples of 4-combining (1), 6-combining (2), 8-combining (3) and 10-combining (4) are shown in FIG. 19B. If the 4-combining are taken, for example, then the number of combinations of the amplifiers used for cancellation is two. The first combination is amplifiers {(1, 2), (3, 4)} and the second combination is amplifiers {(1, 3), (2, 4)}. Thus, a transmission-line length (electric length) between the amplifiers (1, 2) or a transmission-line length between the amplifiers (1, 3) may be defined as a transmission-line length (=xcex/(2(kxe2x88x921)) for canceling the kth harmonic. If the transmission-line length between the amplifiers (1, 2) is set to xcex/8, for example, then a 5th harmonic is canceled out by the amplifiers (1, 2) and a 3rd harmonic is canceled out by the amplifiers (1, 3). In regard to these combinations, the transmission lines for connecting between the amplifiers are equal in electric length to each other.
It is further necessary for the travelling wave power combiner including the even-numbered amplifiers that a plurality of ways to make the combinations of the amplifiers for canceling harmonics exist and the harmonics are canceled out for all possible combinations. If even some of the possible combinations cannot cancel out the harmonics, then a difference in magnitude occurs between the outputs of the fundamental waves and hence the combiners are kept off-balance. In the example illustrated in FIG. 19B, the transmission-line lengths between the respective adjacent amplifiers are set equal to each other. In this case, the harmonics may be canceled out in the case of any combination of other than the adjacent amplifiers.
It is desirable that the combinations of non-adjoining amplifiers are determined according to the transmission-line lengths. In this case, the entire circuit can be constructed in compact form. Further, since the power of the high-order harmonics outputted from the amplifiers is generally high in energy as the order of harmonics decreases, it is desirable to cancel out lower-order harmonics, particularly, the second and third harmonics. It is practical that as will be described later, the third harmonics can be canceled out in the combinations of the non-adjoining amplifiers.
Since the harmonics to be canceled out are kept in balance on an amplitude basis and outputted from the respective amplifiers whose characteristics are the same, they are not outputted to the output terminal of the travelling wave power combiner. Thus, in the power amplifier, the connection of the filter for cutting off the harmonic components to the stage subsequent to the travelling wave power combiner becomes unnecessary, whereby an impairment in the power efficiency due to an insertion loss produced by the insertion of the filter is solved.
These and other objects and many of the attendant advantages of the invention will be readily appreciated as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings.